When I connect the Stereo output from my Yamaha Portable Grand Piano keyboard to a Roland Keyboard amplifier, the left and right channels are quite obviously separated. That can be tested by playing keys above, and below, the Split Point. The obvious nature of the two is heard by the significantly greater loudness of the lower keys to the upper keys. The output connection on the keyboard is stereo and the input on the amp is mono, with the ability to handle 4 mono inputs

The difference is quite distracting when playing thru the amp. This difference is not at all apparent when using the local keyboard amp with the on board speakers

It seems to me that I could conquer my dilemma with a black box with one stereo input, 2 mono outputs, and a volume control. on each of the output lines. The amplifier has four input channels. So the separation is usable. With the "black box" near the keyboard, I could readily adjust the separation to fit the tune, and mood.

I know how to wire the 3 sockets (1 stereo in and 2 mono out, and a common ground to all three). My only question about the black box is how do I create the volume controls? Would an audio pot on each of the output lines work? And, if so, what specs would be appropriate for the output of the Yamaha DGX 202 feeding into the Roland Keyboard amplifier? By specs I mean Ohms and Watts.

Normally you would just make a simple lead, stereo jack to a mono jack - with the left and right channels just shorted together to make it mono. There shouldn't be any need to alter the respective levels.

I do this all the time using a Yamaha stereo keyboard, no problems at all.

Shorting ouputs together is not a good idea. It is fine for inputs.
The outputs should be connected together via resistors near the input.

They aren't speaker outputs (where it would indeed be a bad thing), there are normally internal resistors feeding the line and headphone outputs and it all works perfectly - I've NEVER heard of any problem doing this, and it's perfectly standard practice.

A headphone outputs can be shorted together assuming its
driven via attenuating resistors from the loudspeakers feed.

For a dedicated low impedance heaphone amplifier outputs
and line levels outputs that can drive long cables simply
they should not be shorted together, especially when
you know the two signals are completely different.